We use classical immunogenetic techniques as well as techniques of molecular biology to study the genetics of rabbit immunoglobulins (Igs) and T cell receptors and the regulated expression of the genes that encode these molecules. Basilea rabbits carry a mutation that results in loss of expression of the normal major Ig light chain type (Klb9). We showed that the 3' acceptor splice site of the J kappa-C kappa intron of the kappa 1b9 gene in Basilea rabbits has a G to A transition that results in the use of an AAg/ generated by the mutation and located one nucleotide downstream. The splicing generates a frameshift that results in premature termination of message translation. In addition, the splicing is probably inefficient in that much of the detectable K1 mRNA is not completely processed. These data provide strong support for our earlier suggestion that the splice site mutation accounts for the mutant Basilea phenotype (non- expression of K1b9 light chains). we are investigating why rabbits produce only trace amounts of Igs with light chains of the K2 isotype. We can detect mRNA encoding such light chains in splenocytes from rabbits infected with Trypanosoma equiperdum albeit at 1/100 to 1/1000 the levels in comparable mRNA preparations from Basilea rabbits. We found DNA fragments that bind to the nuclear matrix in a region of the J kappa-C kappa intron upstream of the enhancer that is deleted in the gene for K2 light chains. The functional significance of this deletion can now be evaluated.